This invention relates to semiconductor fabrication methods and apparatus for implementing such methods. More particularly, the present invention relates to metal oxide gate structures for semiconductor devices, e.g., MOSFET devices, memory devices, etc., and other structures including metal oxide dielectric material.
Semiconductor devices such as field effect transistors are commonly used in the electronics industry. Such devices may be formed with extremely small dimensions, such that thousands or even millions of these devices may be formed on a single crystal silicon substrate or xe2x80x9cchipxe2x80x9d and interconnected to perform useful functions in an integrated circuit such as a microprocessor, a memory device, etc. For example, metal oxide semiconductor (MOS) devices are widely used in memory devices that comprise an array of memory cells that include field effect transistors and capacitive structures.
Although transistor design and fabrication are generally complex, a simplified field effect transistor is described below. In such a field effect transistor, a portion of a substrate near the surface is designated as a channel of the transistor. The channel is electrically connected to a source and a drain such that when a voltage difference exists between the source and the drain, current will tend to flow through the channel. The semiconducting characteristics of the channel are altered such that its resistivity may be controlled by the voltage applied to a gate, which generally includes a conductive layer or gate electrode overlying the channel. By changing the voltage on the gate, more or less current can be made to flow through the channel. The gate electrode and the channel are separated by a gate dielectric. Generally, the gate dielectric is insulating, such that between the gate and channel little or no current flows during operation, although tunneling current is observed within certain dielectrics. The gate dielectric allows the gate voltage to induce an electric field in the channel.
Generally, integrated circuit performance may be enhanced by scaling. In other words, performance and density are enhanced by decreasing the size of the individual semiconductor devices on the chip. This has been accomplished by decreasing the thickness of the gate dielectric, thus bringing the gate in closer proximity to the channel. As modem silicon device size becomes smaller or has been scaled to smaller and smaller dimensions, with a corresponding size reduction of the gate length of MOS devices, the gate dielectric thickness has continued to decrease, for example, to less than 2 nm (20 xc3x85) and as thin as 1 nm (10 xc3x85).
However, the most commonly used gate dielectric material, silicon dioxide, exhibits high leakage current density in this thickness range because of direct band-to-band tunneling current or Fowler-Nordheim tunneling current. Further, because such silicon dioxide layers are formed from a few layers of atoms, complex process control is required to repeatably produce such silicon dioxide layers. Further, uniformity of coverage is also critical because device parameters may change dramatically based on the presence or absence of even a single monolayer of dielectric material. Because of the limitations of silicon dioxide, alternative high dielectric constant (K) films such as TiO2, Ta2O5, HfO2, and other high dielectric films have received a lot of interest as substitutions for very thin silicon dioxide gate dielectrics. Such alternate dielectric materials can be formed in a thicker layer than silicon dioxide and yet still produce the same field effect performance. Such performance is often expressed as xe2x80x9cequivalent oxide thickness.xe2x80x9d In other words, although the alternate material layer may be thick, it has the equivalent effect of a much thinner layer of silicon dioxide. Most of the interest in alternate materials for silicon dioxide have employed the use of metal oxides.
Various methods have been described for the formation of metal oxides, e.g., formation of metal oxide gate dielectrics. For example, in Haraguchi et al., xe2x80x9cA TiO2 Gate Insulator of a 1-nm Equivalent Oxide Thickness Deposited by Electron-Beam Evaporation,xe2x80x9d Extended Abstracts of 1999 International Conference on Solid State Devices and Materials, pps. 376-377 (1999), fabrication of thin dielectric films by electron beam evaporation was described. As described in Haraguchi et al., one of the more common methods of forming metal oxide films, e.g., titanium dioxide (TiO2), is by chemical vapor deposition. However, for example, impurities such as carbon and chlorine originating from titanium precursors in such chemical vapor deposition processes may cause undesirable influence on the TiO2 film properties. To achieve the preparation of high purity TiO2 films, electron beam evaporation (as described in Haraguchi et al.) has been used instead of chemical vapor deposition.
For example, as described in Haraguchi et al., electron beam evaporation for forming metal oxides was performed in the ambient of ozone plasma minimizing the effect of oxygen depletion, resulting in pure TiO2 films. Further, by optimizing TiO2 deposition thickness and TiO2 annealing conditions, TiO2 films with 1 nm equivalent oxide thickness which showed low leakage current and interface trap density were realized.
However, even though electron beam evaporation methods have been found to produce metal oxides which show low leakage current and have suitable equivalent oxide thickness, optimization of such film formation processes are necessary. The optical properties for most vacuum evaporated thin films change when the films are exposed to moisture, and they are unstable in air since the properties are dependent on the relative humidity. Such properties are attributed to microstructure of the films, which have been reported to include approximately cylindrical columns several tens of nanometers in diameter with voids between them. As a result, the density of the films is less than that of the bulk material. Upon contact with the moisture, the internal surfaces of the columns adsorb a monolayer of water. On exposure to a humid atmosphere, the voids act as capillaries and fill with water, upon bringing the relative humidity above a certain threshold, which depends upon the diameter of the pores. Consequently, the refractive indices of the films when deposited are less than those of the bulk material and change when the film is exposed to a humid atmosphere. The extent of the change is dependent upon the relative humidity. Typical packing densities for such films have been found to be between 0.75 to 1.0.
Higher packing densities for films and, hence, increased stability were reported to be achieved as described in an article by Martin et al., xe2x80x9cIon-beam-assisted deposition of thin films,xe2x80x9d Applied Optics, Vol. 22, No. 1 Jan. 1, 1983), where the adatoms had greater mobility on the substrate surface. The article indicates they can be produced by heating a substrate or by increasing the energy of the arriving atoms or molecules as occurs in sputtering or ion beam deposition. Additional activation energy can be added to the growing film if it is bombarded with low energy ions during deposition, as reported therein.
In addition, an article by Souche et al., entitled xe2x80x9cVisible and infrared ellipsometry study of ion assisted SiO2 films,xe2x80x9d Thin Solid Films, Vol. 313-314, pps. 676-681 (1998), described the study of oxygen ion-assisted silica thin films by means of in situ visible spectroscopic ellipsometry and infrared spectroscopic ellipsometry in air. The article discusses the transition from porous evaporated films to compact films, with emphasis on compaction of silicon dioxide films by ion-assisted deposition.
Further, ion-assisted deposition of silver thin films was described in an article by Lee et al., entitled xe2x80x9cIon-assisted deposition of silver thin films,xe2x80x9d Thin Solid Films, Vol. 359, pps. 95-97 (2000). The article describes silver films deposited with ion bombardment which are more durable in a humid environment and maintain a higher value of reflectance over time than those deposited without ion bombardment. The effects of ion bombardment was found to reduce the surface roughness and increase the film density. Further, the hardness of the films increased. Yet further, the article described the finding that lattice spacing increased.
The present invention optimizes the formation of high dielectric films using electron beam evaporation. For example, the present invention optimizes such evaporation processes with the use of high purity source materials, use of ion beam bombardment techniques, use of an ozone environment, etc.
A method for use in fabrication of a gate structure according to the present invention includes providing a substrate assembly having a surface located in a vacuum chamber and forming a gate dielectric on the surface. The formation of the gate dielectric comprises forming a metal oxide on at least a portion of the surface of the substrate assembly by electron beam evaporation and generating an ion beam using an inert gas to provide inert gas ions for contacting the metal oxide during formation thereof.
In one embodiment of the method, an environment including oxygen may be provided in the vacuum chamber. The formation of the metal oxide occurs in the oxygen environment. For example, the environment provided may be an ozone environment in the vacuum chamber and/or an ozonizer structure proximate the substrate assembly surface may be used to direct ozone towards the substrate assembly surface.
In other embodiments of the method, the method may include heating the substrate assembly as the metal oxide is formed and/or delaying contact of the inert gas ions with the metal oxide until at least a monolayer of metal oxide is formed.
A method for use in fabrication of a gate structure according to the present invention includes providing a substrate assembly having a surface located in a vacuum chamber and forming a gate dielectric on the surface. The formation of the gate dielectric includes providing an environment including ozone in the vacuum chamber, forming TiO2 on at least a portion of the surface of the substrate assembly by electron beam evaporation in the environment including ozone, and generating an ion beam using an inert gas to provide inert gas ions for contacting the TiO2 during formation thereof.
In one embodiment of the method, forming TiO2 on at least the portion of the surface of the substrate assembly by electron beam evaporation includes directing an electron beam at a high purity TiO2 source material. The high purity source material has a purity of TiO2 that is about 99.999% or greater.
In another embodiment of the method, forming TiO2 on at least the portion of the surface of the substrate assembly by electron beam evaporation includes directing an electron beam at the high purity TiO2 source material such that a deposition rate for TiO2 on the surface of the substrate assembly is about 0.1 nm/second to about 0.2 nm/second.
In other embodiments of the method, forming TiO2 on at least a portion of the surface of the substrate assembly may include forming TiO2 directly on at least a silicon containing portion of the surface of the substrate assembly and/or the method may include forming a conductive gate electrode on the gate dielectric.
Another method for forming a high dielectric constant metal oxide in the fabrication of integrated circuits is described. The method includes providing a substrate assembly having a surface located in a vacuum chamber and forming a metal oxide on at least a portion of the surface of the substrate assembly by evaporating a metal oxide source material using an electron beam. Contact of inert ions with the metal oxide is provided during formation thereof.
In other embodiments of the method, the metal oxide may be at least a portion of a gate dielectric or the metal oxide may be at least a portion of a dielectric material for a capacitor.
A system for use in the fabrication of a gate structure according to the present invention includes a vacuum chamber including a substrate assembly holder adapted to hold a substrate assembly having a surface and an ozonizer apparatus. The ozonizer apparatus includes an ozone source and an ozonizer structure proximate the surface of the substrate assembly in the vacuum chamber. The ozonizer structure has openings adapted to direct ozone towards the surface of the substrate assembly. The system further includes an evaporation apparatus. The evaporation apparatus includes a metal oxide source and an electron beam generation device operable to generate an electron beam that impinges on the metal oxide source to evaporate metal oxide of the metal oxide source for formation of metal oxide on the surface of the substrate assembly. Yet further, the system includes an ion beam apparatus. The ion beam apparatus includes an inert gas source operable to provide an inert gas and an ion gun operable to generate an ion beam using the inert gas and directing the ion beam for contact at the surface of the substrate assembly.
In various embodiments of the system, the metal oxide source may include a high purity source material (e.g., a purity that is about 99.999% or greater); the metal oxide source may include material selected from the group consisting of TiO2, Y2O3, Al2O3, ZrO2, HfO2, Y2O3xe2x80x94ZrO2, ZrSiO4, LaAlO3, and MgAl2O4; and/or the ion beam apparatus may include a controller operable to delay generation of the ion beam until at least a monolayer of metal oxide is formed using the evaporation apparatus.